How can I seamlessly switch between IK/FK?
Under the Rig Main Properties you have the IK/FK slider. Below that you have two options to snap the pose to either the IK or FK position.



p.s. I notice you are using the MetaRig to animate. Generally the MetaRig is used to generate a new rig with all the bells and whistles like IK/FK organised for you.



The 'generation' also creates the Rig Layers which makes switching and controlling easier...